Interferometry is a useful technique for measuring one or more spatial characteristics of a sample, such as a semiconductor wafer or any other semiconductor or non-semiconductor substrate, utilizing information associated with illumination reflected from test surfaces of a sample. As semiconductor fabrication continually requires higher levels of accuracy and precision, improved interferometry techniques are needed to meet the demands of modern fabrication technologies. Phase-shifting interferometry offers many advantages over other interferometry techniques including, but not limited to, higher measurement accuracy, insensitivity to contrast, illumination uniformity, and obtainable phase at fixed grid points.
In phase-shifting interferometry, a time-varying phase shift is applied to illumination between reference surfaces in a phase-shifting interferometry system and test surfaces of a sample. Reflected illumination is then analyzed to determine intensity of the recorded phases. Information associated with spatial characteristics of the sample can be derived accordingly. One method of applying a time-varying phase shift is by physically moving the reference surfaces or the sample to provide phase-shifting by changing the spatial relationship between the test and reference surfaces. However, the physical motion often results in vibrations tending to degrade measurement accuracy.
Alternatively, phase-shifting interferometry systems may rely on wavelength-tunable illumination sources to provide time-varying phase shifts by providing illumination at different wavelengths. Several wavelength-tunable illumination sources are tuned to different wavelengths by varying the applied level of voltage. However, varying the applied level of voltage tends to also affect optical power of the supplied illumination.